The present invention relates to an organic EL device used in a color television, a personal computer, a pachinko machine or the like, and a method of manufacturing the organic EL device. In particular, the present invention relates to a top emission type organic EL device in which the upper electrode has special features, and a method of manufacturing the organic EL device.
Ever since the announcement by C. W. Tang et al. of the Eastman Kodak Company of a high-efficiency organic EL device, which had a two-layer structure, in 1987 (see Tang and Van Slyke, Appl. Phys. Lett., 51, 913 (1987)), various organic EL devices have been developed, and some have been commercialized. In the development of organic EL devices, improving the efficiency of the device is a very important issue from the viewpoint of practical application.
With an organic EL device, light from a light-emitting layer is divided into light that travels directly to the outside, and light that travels to the outside after being reflected. To improve the light emission efficiency, it is necessary to use the reflected waves efficiently. With a so-called ‘bottom emission’ type organic EL device 100 as shown in FIG. 1, the light is extracted from the side of transparent substrate 102. In such a device aluminum, which has a low work function, is often used as cathode 108. Aluminum has high reflectivity and is thus a very suitable metal, and hence the light from the organic EL layer can be reflected and extracted from the side of transparent substrate 102 efficiently. In FIG. 1, the anode is represented by reference numeral 104, and the organic EL layer is represented by reference numeral 106. FIG. 1 shows a situation in which organic EL layer 106 comprises hole injection layer 110, hole transport layer 112, organic light-emitting layer 114, electron transport layer 116, and electron injection layer 118.
In recent years, development of active matrix type devices for organic EL displays has been pursued vigorously. With a bottom emission type active matrix type organic EL display, a TFT substrate is used, and a light-emitting part including the organic EL layer is formed thereon. However, the current state of the art is that there is much fluctuation in the properties of the TFTs and organic EL layer, and various driving circuitry must be added to correct for this fluctuation. This results in complex circuitry. With more complex circuitry, the number of TFTs required to drive each pixel of the organic EL display increases. This increase in the number of TFTs results in an increase in the proportion of the area of the transparent part of the lower electrode that is occupied by the TFTs, which results in a drop in the area for extraction of light. For this reason, a ‘top emission’ type in which the light is extracted from the side of the upper electrode, i.e., the opposite side to the substrate, is advantageous compared with the bottom emission type (see Japanese Patent Application Laid-open No. 2001-176660). Such a top emission type organic EL device is shown in FIG. 2. In FIG. 2, the same reference numerals as in FIG. 1 have been used for corresponding elements. Organic EL device 200 of FIG. 2 has reflective film 202, transparent anode 104, organic EL layer 106 comprising a hole injection layer, a hole transport layer, a light-emitting layer, an electron transport layer and an electron injection layer, and transparent cathode 208, on transparent substrate 102. With the top emission type organic EL device, the light is extracted from the side of transparent cathode 208. As shown in FIG. 2, part 204 of the light emitted from the organic EL layer 106 is extracted to the outside from the side of transparent cathode 208 directly, and part 206 of the light is extracted from the side of transparent cathode 208 after having been reflected by reflective film 202.
With such a top emission type organic EL device, because the light is extracted from the side of the cathode, the cathode must be made of an electrode material that has good electrical conductivity and transparency, and also a low work function. Examples of materials having good electrical conductivity and transparency are indium-tin oxide (ITO) and indium-zinc oxide (IZO), but these materials have a high work function. The idea of depositing a thin layer of an alkali metal, alkaline earth metal or rare earth metal having a low work function as an electron injection layer on the electron transport layer has thus been disclosed (see Japanese Patent Application Laid-open No. 2001-52878). In the situation where such a transparent oxide electrode and electron injection layer are adjacent to one another, atomic diffusion occurs at the interface between the two, which leads to an increase in the interface resistance and a resultant drop in the efficiency.
More recently, a cathode comprising an electron injector having a two-layer cathode with a metal layer having a work function of less than 4 eV or a thin inner layer made of a metal fluoride or a metal oxide and an Al outer layer, an electrically conductive semi-transparent metal layer that augments the electrical conductivity of the electron injector, and a transparent layer having a refractive index of at least 1.2, and also a surface light-emitting organic light-emitting diode using the cathode have been reported (see Japanese Patent Application Laid-open No.10-74586). This report discloses a cathode in which a material having high transmissivity is laminated onto an electrically conductive metal film having low transmissivity. With such a cathode, the transmissivity can be improved for only a specific wavelength and wavelength in the vicinity thereof, but the transmissivity drops for other wavelength regions.
As described above, with a top emission type organic EL device in which the upper electrode is made to be a cathode, there is a problem that the efficiency of light extraction from the organic EL layer is low.